Electric water pump

ABSTRACT

An electric water pump may comprise an impeller rotated by a motor and a casing accommodating the impeller. The impeller may include a plurality of blades. The casing may include a first partial casing, a second partial casing and a tongue portion that extends along the rotation direction of the impeller. The tongue portion may include a first partial tongue portion. formed to be integrated with the first partial casing and a second partial tongue portion formed to be integrated with the second partial casing. A plurality of partial rear edges is disposed at a rear edge of the tongue portion in the rotation direction of the impeller. The plurality of partial rear edges may be positioned to be shifted from each other in the rotation direction of the impeller at a smaller interval than the interval of the blades.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No. 2012-175027 filed on Aug. 7, 2012, the contents of which are hereby incorporated by reference into the present application.

TECHNICAL HELD

The technique disclosed in the present application relates to an electric water pump.

DESCRIPTION OF RELATED ART

Japanese Utility Model Application Publication No. 5-38395 discloses an air blower that blows air with rotation of a fan. In the air blower, with rotation of the fan, air is introduced into a fan scroll unit provided in a case that accommodates the fan. A throat tip positioned at the narrowest gap between the fan scroll unit and the fan is formed in a zigzag form in the extension direction (that is, a direction vertical to the rotation direction of the fan) of the blades of the fan (that is, the throat tip is formed so that the gap between the throat tip and the fan changes in a height direction of the fan). In this configuration, the frequency of a change in the air pressure generated when the blades of the fan pass through the throat tip increases. As a result, the noise generated when the blades of the fan pass through the throat tip becomes relatively high-frequency sound. In this air blower, the noise is absorbed by a noise absorbing material provided to absorb the high-frequency noise.

SUMMARY

In the air blower as above, a configuration in which the frequency of noise increases so that the noise is easily absorbed by the noise absorbing material is employed. In this configuration, it is necessary to provide a noise absorbing material for absorbing high-frequency noise. The present application provides a technique capable of reducing noise even if a new noise absorbing material is not provided.

The description herein discloses an electric water pump. The electric pump may comprise a motor, an impeller rotated by the motor, and a casing that accommodates the impeller. The impeller may include a plurality of blades disposed at an interval in a rotation direction of the impeller. The casing may include a first partial casing, a second partial easing and a tongue portion. The first partial casing may include an inlet port. The second partial casing may be separated from the first partial casing and includes a discharge port. The tongue portion may extend along the rotation direction of the impeller. A gap between an outer circumferential edge of the impeller and an inner circumferential surface of the casing may be smallest between the tongue portion and the impeller. The gap may increase gradually along the rotation direction of the impeller. The gap may be largest in front of the tongue portion. The tongue portion may include a first partial tongue portion formed to be integrated with the first partial casing and a second partial tongue portion formed to be integrated with the second partial casing. A plurality of partial rear edges may be formed at a rear edge of the tongue portion in the rotation direction of the impeller. The plurality of partial rear edges may be positioned to be shifted from each other in the rotation direction of the impeller at a smaller interval than the interval of the blades.

In the electric water pump, water flows into a passage through an inlet port with rotation of the impeller. The water in the passage flows along the rotation direction of the impeller through the passage formed in the gap between the impeller and the casing while being pressurized with rotation of the impeller. That is, the water in the passage flows through the gap (that is, in a direction where the passage area increases) between the impeller and the casing. The water pressurized in the gap between the impeller and the casing is discharged from the discharge port. Since the pressure of the water around the blade changes abruptly when the blade of the impeller passes through a position at which the blade faces the tongue portion where the gap between the impeller and the casing is smallest, pressure pulsation is generated. Due to this, the electric water pump vibrates, which may cause noise.

In the electric water pump, the plurality of partial rear edges formed at the rear edge of the tongue portion in the rotation direction of the impeller is positioned to be shifted from each other in the rotation direction of the impeller. According to this configuration, the pressure pulsation may be reduced as compared to a case where a rear edge of the tongue portion in the rotation direction of the impeller are arranged in a line in the rotation direction of the impeller. Moreover, since the plurality of partial rear edges is positioned to be shifted from each other in the rotation direction of the impeller at a smaller interval than the interval of the blades, different blades may be prevented from simultaneously passing through different partial rear edges. As a result, the pressure pulsation may be reduced. Due to this, vibration in the electric water pump may be reduced, which may cause noise. According to this configuration, since the cause of noise may be suppressed directly, noise may be reduced even when a new noise absorbing material is not provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a longitudinal cross-sectional view of a water pump. FIG. 2 shows an exploded perspective view of a casing. FIG. 3 shows an enlarged view of a portion indicated by III in FIG. 2. FIG. 4 shows a bottom view of an upper casing. FIG. 5 shows a cross-sectional view along line IV-IV in FIG. 1. FIG. 6 shows a view for explaining laser welding. FIG. 7 shows a configuration of a tongue portion of a modification.

DETAILED DESCRIPTION

Major features of the embodiments described below will be described. Technical elements described below are independent technical elements, and may be utilized separately or in all types of combinations.

(1) A first partial tongue portion may include a contacting portion that makes contact with a second partial tongue portion. A positional relationship in a rotation direction of an impeller between the first and second partial tongue portions may be determined by the contacting portion making contact with the second partial tongue portion.

According to this configuration, the tongue portion may be formed with high accuracy by allowing the contacting portion to make contact with the second partial tongue portion.

(2) A second partial casing may include the inner circumferential surface that faces an outer circumferential edge of the impeller and an opening being opened to a first partial casing, and the first partial casing may include a convex portion that protrudes toward the second partial casing and has a shape corresponding to a shape of the inner circumferential surface of the second partial casing.

According to this configuration, the first and second partial casings may be aligned with respect to each other by disposing the convex portion in the inner circumferential surface of the second partial casing.

(3) The first partial casing may include a first adhesion portion that surrounds the convex portion. The second partial casing may include a second adhesion portion that surrounds the opening. The first and second adhesion portions may be welded by laser welding.

According to this configuration, the first partial casing is bonded to the second partial casing by laser welding in such a manner of surrounding the second partial casing. According to this configuration, the first and second partial casings may be appropriately bonded.

(4) A first adhesion portion may be formed of polyphenylene sulfide that does not contain carbon, and a second adhesion portion may be formed of polyphenylene sulfide that contains carbon. A thickness of the first adhesion portion may be 1 mm or smaller. The first and second adhesion portions may be bonded when the second adhesion portion is irradiated with a laser beam from a side of the first adhesion portion opposite to the second adhesion portion so that the second adhesion portion is fused.

According to this configuration, since the first adhesion portion does not contain carbon and has the thickness of 1 mm or smaller, the laser beam may pass through the first adhesion portion. On the other hand, since the second adhesion portion contains carbon, the second adhesion portion is easily fused with the laser beam. According to this configuration, the first and second adhesion portions by fusing the second adhesion portion by laser welding may be bonded.

Representative, non-limiting examples of the present invention will now be described in further detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Furthermore, each of the additional features and teachings disclosed below may be utilized separately or in conjunction with other features and teachings to provide improved electric water pumps, as well as methods for using and manufacturing the same.

Moreover, combinations of features and steps disclosed in the following detailed description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe representative examples of the invention. Furthermore, various features of the above-described and below-described representative examples, as well as the various independent and dependent claims, may be combined in ways that are not specifically and explicitly enumerated in order to provide additional useful embodiments of the present teachings.

All features disclosed in the description and/or the claims are intended to be disclosed separately and independently from each other for the purpose of original written disclosure, as well as for the purpose of restricting the claimed subject matter, independent of the compositions of the features in the embodiments and/or the claims. In addition, all value ranges or indications of groups of entities are intended to disclose every possible intermediate value or intermediate entity for the purpose of original written disclosure, as well as for the purpose of restricting the claimed subject matter.

(Configuration of Water Pump 10)

A water pump 10 is installed in an engine room of a vehicle and is used for circulating coolant that cools an engine, an inverter, and the like. The water pump 10 is an electric pump that is driven with electric power supplied from the outside.

As shown in FIG. 1, the water pump 10 includes a pump unit 20 and a motor unit 40. The outer circumference of the water pump 10 forms a casing 12. The casing 12 includes an upper casing 28 and a lower casing 46. In the present embodiment, an up-down direction is defined when the water pump 10 is in the state shown in FIG. 1. The upper casing 28 blocks an upper opening of the lower casing 46.

(Configuration of Motor Unit 40)

The motor unit 40 is disposed at lower portion of the water pump 10. The motor unit 40 is formed in the lower casing 46. The motor unit 40 configures a brushless motor. The motor unit 40 includes a shaft 16, a rotor 42 of a rotating body 60, and a stator 44. The lower end of the shaft 16 is fixed to the lower casing 46. The shaft 16 extends in the up-down direction within the casing 12, and a distal end thereof reaches the pump unit 20 positioned above the motor unit 40. The rotating body 60 is rotatably attached to the shaft 16. The rotating body 60 includes a rotor 42 and an impeller 26. The rotor 42 is accommodated in the lower casing 46. The rotor 42 has a cylindrical shape. The rotor 42 has a plurality of magnetic poles arranged in the circumferential direction. The impeller 26 and the rotor 42 are integrated with each other. Thus, when the rotor 42 rotates, the impeller 26 also rotates.

The stator 44 is disposed on the outer circumferential side of the rotor 42. Although a longitudinal cross-section of the stator 44 is shown in FIG. 1, oblique parallel lines indicating a cross-section is not depicted to make the drawing of the stator 44 easily visible. The stator 44 is accommodated in the lower casing 46. The stator 44 includes two cores formed by stacking a plurality of core plates. A plurality of teeth wound with coil wires is formed in each core. The distal ends of the teeth face the outer circumferential surface of the rotor 42.

A control circuit 18 that controls supply of electric power to the stator 44 is disposed under the motor unit 40. The control circuit 18 is connected to an external power supply (not shown; for example, a battery mounted on a vehicle) through a terminal 14. The control circuit 18 supplies electric power supplied from the external power supply to the motor unit 40.

(Configuration of Pump Unit 20)

The pump unit 20 is provided above the motor unit 40. The pump unit 20 is provided within the lower casing 46. The upper portion of the pump unit 20 is covered with the upper casing 28. In the pump unit 20, an inlet port 22, a discharge port 23, and a coolant passage 24 are formed by the casing 12. Moreover, the pump unit 20 includes an impeller 26 of the rotating body 60 described later. The impeller 26 is accommodated in the casing 12. As shown in FIG. 5, the impeller 26 has a circular shape when seen from the top of FIG 1. In FIG. 5, a configuration provided in the upper portion of the lower casing 46 is partially omitted. When electric power is supplied to the water pump 10, the impeller 26 rotates in a rotation direction R. A plurality of blades 26 a (seven blades 26 a in the present embodiment) is formed on an upper surface of the impeller 26 at a constant interval in the rotation direction R. The blade 26 a protrudes upward from the bottom plate of the impeller 26. The blade 26 a is curved in the rotation direction R as it advances from the inner circumferential end of the impeller 26 toward the outer circumferential side. Moreover, the thickness of the blade 26 a increases gradually as it advances from the inner circumferential end of the impeller 26 toward the outer circumferential side. The outer circumferential edge of the impeller 26 (that is, an outer circumferential end of the blade 26 a) and an inner circumferential surface 46 b of the lower casing 46 are disposed at an interval. The coolant passage 24 is formed between the outer circumferential edge of the impeller 26 and the inner circumferential surface 46 b.

The coolant passage 24 is formed along the outer circumference (that is, in the rotation direction R) of the impeller 26. The inner circumferential surface 46 b is gradually separated from the outer circumference of the impeller 26 as it advances in the rotation direction R. Thus, a passage area of the coolant passage 24 increases gradually in the rotation direction R. The discharge port 23 is connected at a position where the gap (that is, the passage area of the coolant passage 24) between the outer circumferential edge of the impeller 26 and the inner circumferential surface 46 b is the largest. The discharge port 23 is formed in the lower casing 46. The discharge port 23 extends in a tangential direction of the coolant passage 24 (that is, the impeller 26). The coolant passage 24 surrounds in the circumferential direction of the impeller 26. When the coolant passage 24 is seen along the rotation direction R, the position at which the passage area of the coolant passage 24 is the largest and the position at which the passage area is the smallest are adjacent to each other.

The casing 12 includes a tongue portion 27 that is formed at the position, at which the passage area of the coolant passage 24 is the smallest, so as to extend along the rotation direction R. In other words, the passage area of the contacting portion 24 (that is, the gap between the outer circumferential edge of the impeller 26 and the inner circumferential surface 46 b of the lower casing 46) decreases due to the tongue portion 27. A portion of the inner circumferential surface 46 b positioned in the tongue portion 27 has approximately the same height as the height of the other portion in the up-down direction of the inner circumferential surface 46 b. The thickness of the tongue portion 27 gradually increases as it advances from a rear edge 50 in the rotation direction R toward the front side. As will be described later, referring FIG. 3, the tongue portion 27 includes a partial tongue portion 28 c and a partial tongue portion 46 e. In addition, the tongue portion 27 of in FIG. 3 is plane-symmetrical to the tongue portion 27 of FIG. 5. Further, the casing 12 of FIG. 2 is plane-symmetrical to the casing 12 of FIGS. 1 4, 5.

The rear edge 50 of the tongue portion 27 extends from the lower end of the coolant passage 24 to the upper end. The rear edge 50 includes partial rear edges 52 and 54. The partial rear edges 52 and 54 extend in parallel (that is, in the up-down direction) to the shaft 16. The partial rear edges 52 and 54 extend linearly. The partial rear edge 52 is positioned to be shifted toward the front side in the rotation direction R (that is, the rotation direction R is direction from left to right in FIG. 3) more than the partial rear edge 54. The distance between the partial rear edges 52 and 54 in the rotation direction R is smaller than the distance between adjacent two blades 26 a in the rotation direction R.

(Configuration of Upper Casing 28 and Lower Casing 46)

As shown in FIG. 2, the upper casing 28 is formed of polyphenylene sulfide (PPS) that does not contain carbon. In FIG. 2, portions of the lower casing 46 constituting the pump unit 20 are depicted, and portions that constitute the motor unit 40 are not depicted. The upper casing 28 includes the inlet port 22, a contacting portion 28 a, a spigot joint portion 28 b, and the partial tongue portion 28 c. The inlet port 22 is provided on the extension line of the shaft 16. The contacting portion 28 a has a flat ring shape that surrounds the inlet port 22 and the spigot joint portion 28 b. The thickness of the contacting portion 28 a is 1 mm or smaller. The spigot joint portion 28 b is disposed on the lower surface of the upper casing 28. The spigot joint portion 28 b protrudes toward the lower casing 46 (that is, downward). The spigot joint portion 28 b has an outer circumferential surface that extends along the inner circumferential surface 46 b of which the distance to the outer circumferential edge of the circular impeller 26 changes gradually. The tongue portion 28 c is disposed on the lower surface (that is, the spigot joint portion 28 b) of the upper casing 28. The tongue portion 28 c protrudes downward from the lower surface of the upper casing 28. As shown in FIG. 3, the tongue portion 28 c includes the partial rear edge 52 that is disposed at the rear edge in the rotation direction R.

As shown in FIG. 2, the lower casing 46 is formed of PPS that contains carbon. That is, the upper casing 28 and the lower casing 46 are formed of PPS in which deterioration due to water absorption is very low. The lower casing 46 includes the discharge port 23, a contacting portion 46 a, an inner circumferential surface 46 b, and the partial tongue portion 46 c. The contacting portion 46 a is formed at the upper end of the lower casing 46. The contacting portion 46 a has a flat ring shape corresponding to the shape of the contacting portion 28 a. The partial tongue portion 46 c protrudes upward (that is, toward the upper casing 28) from the bottom surface of the coolant passage 24 that is formed in the lower casing 46. The partial tongue portion 46 c includes the partial rear edge 54 that is disposed at the rear edge in the rotation direction R.

The upper casing 28 is assembled with the lower casing 46 so that the partial tongue portion 28 c makes contact with the partial tongue portion 46 c. In this case, the spigot joint portion 28 b is inserted into an opening of the inner circumferential surface 46 b. As shown in FIG. 3, the partial tongue portion 46 c includes a contacting portion 46 d that is disposed at an upper end of the partial tongue portion 46 c so as to protrude upward. The contacting portion 46 d is positioned at the rear end of the partial tongue portion 46 c. In a state where the upper casing 28 and the lower casing 46 are assembled together, the partial tongue portion 28 c is in contact with the partial tongue portion 46 c. Specifically, the lower surface of the partial tongue portion 28 c is in contact with the upper surface of the lower casing 46. Moreover, the contacting portion 28 d of the partial tongue portion 28 c is in contact with the contacting portion 46 d. The contacting portion 28 d makes contact with the contacting portion 46 d from the front side in the rotation direction R. Due to this, the partial tongue portion 28 c is restricted from moving toward the opposite side in the rotation direction R in relation to the partial tongue portion 46 c. Further, a front edge 28 e of the partial tongue portion 28 c is in contact with the lower easing 46. The front edge 28 e makes contact with the lower casing 46 from the rear side in the rotation direction R. Due to this, the partial tongue portion 28 c is restricted from moving in the rotation direction R in relation to the partial tongue portion 46 c. The positional relationship in the rotation direction R between the partial tongue portions 28 c and 46 c is determined when the contacting portion 28 d makes contact with the contacting portion 46 d and the front edge 28 e makes contact with the lower casing 46.

When the upper casing 28 and the lower casing 46 are assembled together, the spigot joint portion 28 b makes contact with the inner circumferential surface 46 b so that the upper casing 28 and the lower casing 46 can be aligned with respect to each other. Moreover, when the upper casing 28 and the lower casing 46 are assembled together, the contacting portions 28 a and 46 a make contact with each other. Due to this, the positional relationship in the up-down direction between the upper casing 28 and the lower casing 46 is determined.

As shown in FIG. 6, after the upper casing 28 and the lower casing 46 are assembled together, the contacting portion 46 a is irradiated with a laser beam X from the upper side of the contacting portion 28 a so that the contacting portions 28 a and 46 a are bonded. In FIG. 6, although two laser beams X are illustrated, one laser beam is emitted so as to surround the contacting portion 28 a. The laser beam X passes through the contacting portion 28 a to reach the contacting portion 46 a. No carbon is contained in the PPS that constitutes the contacting portion 28 a. Moreover, the thickness of the contacting portion 28 a is 1 mm or smaller. Thus, even when the laser beam X is emitted from the upper side of the contacting portion 28 a, the laser beam X can easily pass through the contacting portion 28 a. Moreover, carbon is contained in the PPS that constitutes the contacting portion 46 a, Thus, the contacting portion 46 a is easily fused with the laser beam X. According to this configuration, by emitting the laser beam X from the upper side of the contacting portion 28 a, the contacting portion 46 a can be easily fused, and the contacting portions 28 a and 46 a can be welded together.

In addition, the contacting portions 28 a and 46 a are disposed above the discharge port 23. Thus, the discharge port 23 does not interfere with the emitted laser beam X. Further, the contacting portions 28 a and 46 a re positioned above the impeller 26. The pressure from the coolant is applied to only the lower surface of the upper casing 28 close to the impeller 26. According to this configuration, the area of the upper casing 28 that receives the pressure from the coolant can be decreased as compared to a ease where the adhesion portion between the upper casing 28 and the lower casing 46 is positioned below the impeller 26. As a result, a load applied from the coolant in the pump unit 20 to the adhesion portion between the contacting portions 28 a and 46 a can be reduced.

In addition, the contacting portions 28 a and 46 a are formed to be flat. Thus, it is possible to easily secure adhesion between the contacting portions 28 a and 46 a as compared to a case where the adhesion portion between the upper casing 28 and the lower casing 46 extends in a cylindrical fowl along the outer circumference of the pump unit 20. Further, it is possible to decrease the height of the water pump 10 as compared to a case where the adhesion portion between the upper casing 28 and the lower casing 46 is provided along the outer circumference of the pump unit 20.

(Operation of Water Pump 10)

Next, the operation of the water pump 10 will be described. When electric power is supplied from the external power supply to the water pump 10 through the terminal 14, the control circuit 18 applies a voltage to the coil wires of the stator 44 in a predetermined order. As a result, a voltage of the same phase (that is, any one of the U, V, and W phases) is applied to two of the teeth of the stator 44 facing each other with the shaft 16 interposed. That is, the motor unit 40 is a three-phase AC motor.

When a voltage is applied to the stator 44, the teeth are magnetized. When the teeth are magnetized, a permanent magnet of the rotor 42 is attracted toward the magnetized teeth. Due to this, the rotating body 60 rotates around the shaft 16. The coolant is absorbed into the pump unit 20 through the inlet port 22 with rotation of the impeller 26 and flows into the coolant passage 24. The coolant in the coolant passage 24 flows in the rotation direction R through the coolant passage 24 while being pressurized with rotation of the impeller 26 and is discharged out of the coolant passage 24 through the discharge port 23.

As shown in FIG. 5, during the period when the impeller 26 is rotating, the blades 26 a pass through the tongue portion 27 at a constant interval. As the blades 26 a pass through the tongue portion 27, the blades 26 a move from a position where high-pressure coolant is present to a position where low-pressure coolant is present. That is, the pressure of the coolant present around the blades 26 a changes abruptly. Thus, when the blades 26 a pass through the tongue portion 27, pressure pulsation is generated. When pressure pulsation is generated, vibration is generated in the water pump 10. As a result, noise may be generated, and adjacent other components or the water pump 10 itself may be damaged.

In the water pump 10, the rear edge 50 of the tongue portion 27 is divided into the partial rear edges 52 and 54. Moreover, the partial rear edges 52 and 54 are disposed at positions shifted from each other in relation to the rotation direction R. As a result, when the blades 26 a pass through the tongue portion 27, the blades 26 a first pass through the partial rear edge 54 and then pass through the partial rear edge 52. According to this configuration, it is possible to reduce pressure pulsation as compared to a case where the rear edge 50 of the tongue portion 27 extends linearly in the up-down direction. Due to this, the vibration of the water pump 10 is reduced, and noise or the like resulting from the vibration is reduced. In the water pump 10, it is possible to reduce noise or the like without providing a noise absorbing material or the like for absorbing noise. Moreover, the distance between the partial rear edges 52 and 54 in the rotation direction R is smaller than the distance between adjacent blades 26 a in the rotation direction. Thus, it is possible to obviate a situation in which a second blade 26 a different from a first blade 26 a passes through the partial rear edge 54 at the same timing when the first blade 26 a passes through the partial rear edge 52.

(Modifications)

(1) The configuration of the partial tongue portions 28 c and 46 c is not limited to the above embodiment. For example, as shown in FIG. 7, the partial tongue portion 28 c may include a contacting portion 28 f that protrudes downward. The contacting portion 28 f may be positioned at the rear end of the partial tongue portion 28 c. The partial tongue portion 46 c may include a contacting portion 46 f that protrudes upward. The contacting portion 46 f may be positioned on the front side of the partial tongue portion 46 c. In a state where the upper casing 28 and the lower casing 46 are assembled together, the contacting portion 28 f may make contact with the contacting portion 46 f from the rear side in the rotation direction R. Due to this, the partial tongue portion 28 c is restricted from moving toward the opposite side in the rotation direction R in relation to the partial tongue portion 46 c.

(2) In the above embodiment, the rear edge 50 of the tongue portion 27 is divided into two partial rear edges 52 and 54. However, the rear edge 50 of the tongue portion 27 may be divided into three or more partial rear edges. In this case, each partial rear edge may extend linearly in the up-down direction. Moreover, the partial rear edges may be disposed to be shifted from each other in the rotation direction R.

(3) In the above embodiment, the partial tongue portion 28 c includes the partial rear edge 52, and the partial tongue portion 46 c includes the partial rear edge 54. That is, the rear edges of the partial tongue portions 28 c and 46 c are formed on one straight line. However, at least one of the partial tongue portions 28 c and 46 c may include two or more partial rear edges that are positioned to be shifted from each other along the rotation direction R.

(4) The technique disclosed in the present application can be used in an electric pump for pumping water (for example, hot water or the like) other than coolant. 

What is claimed is:
 1. An electric water pump comprising: a motor; an impeller rotated by the motor; and a casing that accommodates the impeller, wherein the impeller includes a plurality of blades disposed at an interval in a rotation direction of the impeller, the casing includes: a first partial casing that includes an inlet port; a second partial casing that is separated from the first partial casing and includes a discharge port; and a tongue portion that extends along the rotation direction of the impeller, a gap between an outer circumferential edge of the impeller and an inner circumferential surface of the casing is smallest between the tongue portion and the impeller, increases gradually along the rotation direction of the impeller, and is largest in front of the tongue portion, the tongue portion includes a first partial tongue portion fbmied to be integrated with the first partial casing and a second partial tongue portion formed to be integrated with the second partial casing, a plurality of partial rear edges is disposed at a rear edge of the tongue portion in the rotation direction of the impeller, and the plurality of partial rear edges is positioned to be shifted from each other in the rotation direction of the impeller at a smaller interval than the interval of the blades.
 2. The electric water pump according to claim 1, wherein the first partial tongue portion includes a contacting portion that makes contact with the second partial tongue portion, and a positional relationship in the rotation direction of the impeller between the first and second partial tongue portions is determined by the contacting portion making contact with the second partial tongue portion.
 3. The electric water pump according to claim 1, wherein the second partial casing includes an inner circumferential surface that s the outer circumferential edge of the impeller and an opening being opened to the first partial casing, and the first partial casing includes a convex portion that protrudes toward the second partial casing and has a shape corresponding to a shape of the inner circumferential surface of the second partial casing.
 4. The electric water pump according to claim 3, wherein the first partial casing includes a first adhesion portion that surrounds the convex portion, the second partial casing includes a second adhesion portion that surrounds the opening, and the first and second adhesion portions are welded by laser welding.
 5. The electric water pump according to claim 4, wherein the first adhesion portion is formed of polyphenylene sulfide that does not contain carbon, the second adhesion portion is formed of polyphenylene sulfide that contains carbon, a thickness of the first adhesion portion is 1 mm or smaller, and the first and second adhesion portions are bonded when the second adhesion portion is irradiated with a laser beam from a side of the first adhesion portion opposite to the second adhesion portion so that the second adhesion portion is fused. 